1. Field of the Invention
This invention relates to Ca-containing zeolite A for the enrichment of oxygen from air, a zeolitic granulate containing SiO.sub.2 as a binder and a zeolite component comprising no CO.sub.3 structures at the exchanged calcium cations.
1. BACKGROUND INFORMATION
The specific energy requirement for the production of oxygen by low-temperature air separation increases greatly for small capacities. In relatively small amounts, oxygen may advantageously be obtained by adsorption air separation processes. The purities obtainable are generally lower than those of the oxygen emanating from the low-temperature separation of air. The adsorptive processes operate at ambient temperature. The necessary equipment is often transportable and is distinguished by short start-up times and minimal handling (cf., for example, G. Reiss, Chem. Ind., XXXV/November, 1983).
In processes for adsorptive separation of air into an oxygen-rich phase and a nitrogen-rich phase, use is made of the greater affinity of the nitrogen for adsorption to the adsorbent compared with the oxygen. The N.sub.2 /O.sub.2 adsorption isotherms show that, for example, Ca zeolite A having a pore diameter of 5 .ANG. adsorbs more nitrogen than oxygen. However, the difference between nitrogen and oxygen is not so great that satisfactory separation of the nitrogen and oxygen in the air appears possible.
The effectiveness of adsorptive air separation can be considerably increased by using a special adsorption process known as pressure change adsorption (PCA). Pressure change adsorption processes are always used when the component to be removed from the crude product gas is present in a relatively high concentration, for example, above 1% by volume, or is inadequately adsorbed to the adsorbent, so that large adsorption units and large quantities of regeneration gas are required for thermal regeneration. In general, the adsorptive separation takes place at a higher pressure than the desorption of the adsorbed components following the adsorption step.
In most cases, desorption is assisted by rinsing of the adsorbent with part of the product gas, for example, in the recovery of nitrogen from combustion gases or drying of gases.
The oxygen enrichment of air occupies a special position in relation to other PCA processes because, in addition to nitrogen, the oxygen and argon in the air are also adsorbed on the molecular sieve zeolites used for this purpose. Accordingly, it is not possible to adsorb only the nitrogen and to recover all the oxygen of the crude product air. Since argon is adsorbed as weakly as oxygen, oxygen purities of 95%, remainder (5%) argon and nitrogen, are obtained in the oxygen enrichment of air.
The oxygen pressure change adsorption process can be divided into three steps:
adsorption with simultaneous recovery of the unadsorbed phase, i.e., oxygen PA0 desorption of the adsorbed phase, in this case nitrogen, carbon dioxide and water of the air, at a pressure below the adsorption pressure, with and without rinsing gas PA0 filling of the adsorber to the adsorption pressure; gas separation may actually take place during this filling process. PA0 adsorption at normal pressure desorption at a reduced pressure of 50-250 mbar PA0 adsorption at an excess pressure of 2-4 bar (abs) desorption at ambient pressure PA0 adsorption at 2-4 bar (abs) desorption at a reduced pressure of 50-500 mbar. PA0 filling only with product (countercurrent) PA0 filling with product (countercurent) and with air (co-current, at different times or simultaneous) PA0 filling with air (co-current, but only when the adsorption exit side has been rinsed beforehand with O.sub.2 product)
All the processes involved take place adiabatically at ambient temperature. Adsorption takes place at a relatively high pressure, while desorption takes place at reduced pressure in the absence of rinsing gas. Refilling is achieved with produced oxygen.
In accordance with the prior art, oxygen PCA plants have the following four principal features:
1. Number of adsorbers
Outwardly, the plants are distinguished by the number of adsorbers to some of which gas holders are connected. In general, the specific energy consumption is lower, the larger the number of adsorbers.
2. Pressure range and cycle time
Major differences exist in the pressure change ranges with the following variants:
For normal-pressure plants, the cycle time per adsorber is 0.5 to 1.5 minutes and, for excess-pressure systems, from 1 to 3 minutes.
3. Partial step
The key to the effectiveness of the oxygen PCA processes is the filling step after desorption. It is important that as little nitrogen as possible be adsorbed on the adsorption exit side before the adsorption step, because this preadsorbed nitrogen can considerably reduce the quality of the O.sub.2 product which is influenced by the filling step or by rinsing with product oxygen. Filling steps may be differentiated as follows:
4. Pre-drying
The predrying of the air must be satisfactory because a CO.sub.2 /H.sub.2 O front advancing in the adsorber destroys the N.sub.2 /O.sub.2 separation.
The pre-purification and N.sub.2 /O.sub.2 separation take place in the same adsorber, a layer of suitable zeolites or other drying agents being arranged on the air entry side.
As already mentioned, zeolitic molecular sieves are used as adsorbents for N.sub.2 /O.sub.2 separation by the PCA process. The properties of the zeolite itself can influence the size and energy demand of O.sub.2 PCA plants in various ways. Two principal requirements are, on the one hand, high nitrogen adsorption with minimal oxygen adsorption and good diffusion during adsorption and desorption.
To achieve high nitrogen adsorption, calcium-exchanged zeolite A is normally used for PCA plants. Particular importance is attributed to the degree of exchange for CaO/mole Al.sub.2 O.sub.3 in the zeolite A. The degree of exchange in the zeolite may vary from 0 to 1.0 mole CaO/mole Al.sub.2 O.sub.3. The adsorption of N.sub.2 in the zeolite increases with increasing degree of exchange for CaO. The degree of exchange is normally 0.4 to 1 mole CaO/mole Al.sub.2 O.sub.3.
For industrial adsorbers, the zeolite is used in granulated form. There are various known processes for the production of granulates. The Ca-exchanged zeolite A powder may be mixed with binders based on clay minerals, such as for example attapulgites, bentonites, sepiolites, kaolinites, ball clays, fireclays or the like, in quantities by weight of 10 to 30% and prefreably 15 to 25% and, after the necessary addition of liquid, the resulting mixtures may be processed into shaped elements in suitable granulating machines such as, for example, roll granulators, extruders, mixing granulators, ring edge-runner presses, extrusion presses or the like. Pan granulators or granulating drums may also be used.
In addition, other binders, such as, for example, Al.sub.2 O.sub.3, SiO.sub.2 or the like, may be used as binders.
Granulating to form beads containing SiO.sub.2 as binder is preferably carried out by a process of the type described in DE-OS 3 401 485.
This process gives and SiO.sub.2 -bound granulate of high macroporosity which is a prerequisite for the use of the granulate in the PCA process.
In addition to the degree of exchange of CaO, the type of activation and the crystallinity of the zeolitic material and also the accessibility of the inner-crystalline adsorption sites are of considerable importance for good oxygen enrichment.